Acoustic isolator downhole applications

ABSTRACT

A plurality of heavy mass irregularities attached to an inner wall of the drill collar attenuate waves traveling through the collar. The plurality of heavy mass irregularities are spaced and sized for the maximum attenuation of acoustic pulses in a predetermined frequency range. The mass irregularities may be rings secured to the inner surface of the collar by neck pieces, extending outwardly from the outer circumference of the ring. The mass irregularities may be made of steel or tungsten and are between six and ten in number. The spacing of the irregularities may lie between twelve and fourteen centimeters. A center pipe may be included to isolate the irregularities from the fluid flow associated with the drilling operation. The pipe may be of a soft material such as rubber to reduce transfer of acoustic noise along the drill string. The irregularities may be in an oil based fluid with the pipe fitting closely in the center of the rings. In another embodiment of the invention, each of the mass irregularities is attached to the drill collar over substantially the entire length of the mass irregularity, enabling the attenuation of high frequencies. In yet another embodiment of the invention, the attenuator comprises a substantially cylindrical body with a plurality of recesses on the inside and/or outside of the cylindrica body, with the length of the recesses selected to provide attenuation within a specified band. In another embodiment of the invention, the attenuator comprises a plurality of sections each having an inner diameter and an outer diameter, each section acting like a waveguide with an associated passband and reject-band.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 09/583,258, now U.S. Pat. No. 6,615,949 filed on May 31, 2000,which claimed priority from U.S. Provisional Patent Application Ser. No.60/137,388 filed on Jun. 3, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to logging while drilling apparatus andmore particularly to acoustic logging while drilling apparatus andattenuation of acoustic pulses that travel parallel to the direction ofdrilling.

2. Related Prior Art

To obtain hydrocarbons such as oil and gas, wells or wellbores aredrilled into the ground through hydrocarbon-bearing subsurfaceformations. Currently, much current drilling activity involves not onlyvertical wells but also drilling horizontal wells. In drilling,information from the well itself must be obtained. While seismic datahas provided information as to the area to drill and approximate depthof a pay zone, the seismic information can be not totally reliable atgreat depths. To support the data, information is obtained whiledrilling through logging while drilling or measuring while drilling(MWD) devices. Logging or measuring while drilling has been a procedurein use for many years. This procedure is preferred by drillers becauseit can be accomplished without having to stop drilling to log a hole.This is primarily due to the fact that logging an unfinished hole, priorto setting casing if necessary, can lead to washouts, damaging thedrilling work that has already been done. This can stall the completionof the well and delay production. Further, this information can beuseful while the well is being drilled to make direction changesimmediately.

Advances in the MWD measurements and drill bit steering systems placedin the drill string enable drilling of the horizontal boreholes withenhanced efficiency and greater success. Recently, horizontal boreholes,extending several thousand meters (“extended reach” boreholes), havebeen drilled to access hydrocarbon reserves at reservoir flanks and todevelop satellite fields from existing offshore platforms. Even morerecently, attempts have been made to drill boreholes corresponding tothree-dimensional borehole profiles. Such borehole profiles ofteninclude several builds and turns along the drill path. Such threedimensional borehole profiles allow hydrocarbon recovery from multipleformations and allow optimal placement of wellbores in geologicallyintricate formations.

Hydrocarbon recovery can be maximized by drilling the horizontal andcomplex wells along optimal locations within the hydrocarbon-producingformations. Crucial to the success of these wells is establishingreliable stratigraphic position control while landing the well into thetarget formation and properly navigating the drill bit through theformation during drilling. In order to achieve such well profiles, it isimportant to determine the true location of the drill bit relative tothe formation bed boundaries and boundaries between the various fluids,such as the oil, gas and water. Lack of such information can lead tosevere “dogleg” paths along the borehole resulting from hole or drillpath corrections to find or to reenter the pay zones. Such well profilesusually limit the horizontal reach and the final well length exposed tothe reservoir. Optimization of the borehole location within theformation also can have a substantial impact on maximizing productionrates and minimizing gas and water coning problems. Steering efficiencyand geological positioning are considered in the industry among thegreatest limitations of the current drilling systems for drillinghorizontal and complex wells. Availability of relatively precisethree-dimensional subsurface seismic maps, location of the drillingassembly relative to the bed boundaries of the formation around thedrilling assembly can greatly enhance the chances of drilling boreholesfor maximum recovery. Prior art down hole devices lack in providing suchinformation during drilling of the boreholes.

Modem directional drilling systems usually employ a drill string havinga drill bit at the bottom that is rotated by a drill motor (commonlyreferred to as the “mud motor”). A plurality of sensors and MWD devicesare placed in close proximity to the drill bit to measure certaindrilling, borehole and formation evaluation parameters. Such parametersare then utilized to navigate the drill bit along a desired drill path.Typically, sensors for measuring downhole temperature and pressure,azimuth and inclination measuring devices and a formation resistivitymeasuring device are employed to determine the drill string andborehole-related parameters. The resistivity measurements are used todetermine the presence of hydrocarbons against water around and/or ashort distance in front of the drill bit. Resistivity measurements aremost commonly utilized to navigate the drill bit. However, the depth ofinvestigation of the resistivity devices usually extends only two tothree meters and resistivity measurements do not provide bed boundaryinformation relative to the downhole subassembly. Furthermore, thelocation of the resistivity device is determined by some depth measuringapparatus deployed on the surface which has a margin of error frequentlygreater than the depth of investigation of the resistivity devices.Thus, it is desirable to have a downhole system which can accurately mapthe bed boundaries around the downhole subassembly so that the drillstring may be steered to obtain optimal borehole trajectories.

The relative position uncertainty of the wellbore being drilled and thecritical near-wellbore bed boundary or contact is defined by theaccuracy of the MWD directional survey tools and the formation dipuncertainty. MWD tools may be deployed to measure the earth's gravityand magnetic field to determine the inclination and azimuth. Knowledgeof the course and position of the wellbore depends entirely on these twoangles. Under normal conditions, the inclination measurement accuracy isapproximately plus or minus two tenths of a degree. Such an errortranslates into a target location uncertainty of about three meters perone thousand meters along the borehole. Additionally, dip ratevariations of several degrees are common. The optimal placement of theborehole is thus very difficult to obtain based on the currentlyavailable MWD measurements, particularly in thin pay zones, dippingformations and complex wellbore designs.

Until recently, logging while drilling has been limited to resistivitylogs, gamma logs, neutron logs and other non-acoustic logs sinceacoustic noise caused by drilling and acoustic pulses traveling upstringfrom the transmitter has presented problems in accurate detection anddelineation. These problems cannot be easily isolated by arrival timesince the acoustic pulses are generated and detected continuously.Recently, the use of acoustic sensors having a relatively short spacingbetween the receivers and the transmitter to determine the formation bedboundaries around the downhole subassembly has been used. An essentialelement in determining the bed boundaries is the determination of thetravel time of the reflection acoustic signals from the bed boundariesor other interface anomalies. A prior art proposal has been to utilizeestimates of the acoustic velocities obtained from prior seismic data oroffset wells. Such acoustic velocities are not very precise because theyare estimates of actual formation acoustic velocities. Also, since thedepth measurements can be off by several meters from the true depth ofthe downhole subassembly, it is highly desirable to utilize actualacoustic formation velocities determined downhole during the drillingoperations to locate bed boundaries relative to the drill bit locationin the wellbore.

Additionally, for acoustic or sonic sensor measurements, the mostsignificant noise source is acoustic signals traveling from the sourceto the receivers via the metallic tool housing and those travelingthrough the mud column surrounding the downhole subassembly (tube wavesand body waves). In some applications acoustic sensor designs are usedto achieve a certain amount of directivity of signals. A transmittercoupling scheme with signal processing method may be used for reducingthe effects of the tube wave and the body waves. Such methods, however,alone do not provide sufficient reduction of the tube and body waveeffects, especially due to strong direct coupling of the acousticsignals between the transmitters and their associated receivers.

Some U.S. patents representative of the current art in determiningsubsurface formations are as follows.

U.S. Pat. No. 4,020,452, titled “Apparatus For Use in InvestigatingEarth Formations”, issued to Jean-Claude Trouiller, et al., relates toan apparatus for mechanically filtering acoustic pulses in a welllogging tool. This apparatus includes of a substantially rigid memberhaving interruptions in the longitudinal continuity of the member. Theseinterruptions provide tortuous paths for the passage of acoustic energyalong the member. A plurality of masses are periodically spaced alongthe interior of the member and are each mechanically integral withopposite sides of the member at locations chosen to enable the memberand masses to cooperate as a mechanical filter. By so doing, thestructure made of the member and masses will have good acoustic delayand attenuation characteristics as well as good mechanicalcharacteristics.

U.S. Pat. No. 5,043,952, titled “Monopole Transmitter For a Sonic WellTool”, issued to David C. Hoyle, et al., relates to a monopoletransmitter for a sonic tool which includes an axial tube, apiezoceramic cylinder surrounding the axial tube, an endcap disposed ateach end of and firmly contacting the cylinder, and an apparatus forholding the endcaps firmly against the axial tube. The endcaps firmlycontact the axial tube without simultaneously contacting an upperbulkhead. The apparatus may include spring washers disposed between thebulkhead and at least one endcap, or it may include a spring disposedbetween a nodal mount and each endcap. A nodal mounting tube may bedisposed around the axial tube, a ring being disposed at each end of thenodal mounting tube, each ring being disposed outside of the cylinderfor biasing the endcaps in tension against a ring thereby holding eachendcap firmly in contact against the axial tube.

U.S. Pat. No. 5,510,582, titled “Acoustic Attenuator, Well LoggingApparatus and Method of Well Logging”, issued to James R. Birchak, etal., relates to a sonic well tool for performing acoustic investigationsof subsurface geological formations penetrated by a borehole. The welltool generally includes a longitudinally extending body for positioningin the borehole. The tool also includes a transmitter supported by thebody for transmitting acoustic energy and a receiver supported by thebody for receiving acoustic energy. The tool includes an acousticattenuation section positioned on the body between the transmitter andthe receiver. This section includes one or more cavities defined by thebody, inertial mass members positioned inside the cavities in a suitablemanner to form a gap between the wall of the cavity and the inertialmass members, and an acoustical attenuation fluid in the gap. The methodfor attenuating sonic waves generally includes transmitting a sonic wavefrom the transmitter to the tool, passing the sonic wave through theacoustic attenuation section, and receiving attenuated wave at thereceivers.

U.S. Pat. No. 5,036,945, titled “Sonic Well Tool Transmitter ReceiverArray Including an Attenuation and Delay Apparatus”, issued to David C.Hoyle, et al., relates to a sonic well tool that includes a transmitterarray having at least one monopole transmitter and at least one dipoletransmitter and a receiver array for receiving sonic pressure wavesignals from a surrounding borehole formation. A first attenuation anddelay apparatus is positioned above the receiver array and a secondattenuation and delay apparatus is positioned below the receiver arrayin the sonic well tool. The first attenuation and delay apparatusincludes an attenuation member comprising a plurality of interleavedrubber and metal like washers for attenuating compressional and flexuralwaves propagating along a metal center support rod to the receiver arrayand an inner housing comprising a bellows section having a corrugatedshape and a thin transverse dimension for delaying the propagation ofcompressional and flexural waves along the inner housing to the receiverarray. The second attenuation and delay apparatus includes a pluralityof mass loading rings surrounding the outer housing of the sonic welltool for attenuating the flexural waves propagating up the outer housingfrom a sonic,transmitter ad a further inner housing including a furtherbellows section having a corrugated shape and a thin transversedimension for delaying the propagation of compressional and flexuralwaves up the tool, along the inner housing, to the receiver array. Thesonic well tool also includes a differential volume compensator forchanging the quantity of oil encapsulated in the sonic well tool inaccordance with changes in oil volume and changes in boreholetemperature and pressure. The receiver array includes a plurality ofhydrophone sets, each hydrophone set including at least one pair andpreferably two pair of hydrophones disposed in a cross section of thetool, one hydrophone of a pair being disposed opposite the otherhydrophone of the pair in the cross section.

U.S. patent application Ser. No. 09/201,988, now U.S. Pat. No. 6,082,484to Molz & Dubinsky, having the same assignee as the present inventiondiscloses the use of a section of a drill collar that has a plurality ofshaped cavities filled with oil. The passage of an acoustic wave sets upa resonance of the fluid in the shaped cavity. The frequency ofresonance depends upon the shape and size of the cavity and theproperties of the fluid in the cavity. In one embodiment of theinvention, the cavities are spherical. Another embodiment of theinvention uses cylindrical cavities with a piston restrained by a springwithin the cavity. Changing the spring constant provides additionalcontrol over the frequencies that are attenuated. The Molz patent alsodiscloses the use of segmented isolators in which the drill collarsection is filled with layers of a composite material in which thelayers have a different density. The thicknesses of the individuallayers is selected to attenuate certain frequencies.

SUMMARY OF THE INVENTION

The present invention provides a system and method for attenuation ofacoustic waves that travel through a drill collar in a logging whiledrilling operation. The system includes a plurality of heavy massesattached to an inner wall of the drill collar. The heavy massesconstitute mass discontinuities that attenuate waves traveling throughthe drill collar. In one embodiment of the invention, the massdiscontinuities are rings and attachment is done by neck pieces. Theseneck pieces extend out from the outer circumference of the rings and maybe an original outer circumference of the ring that has been milled downby cutting out portions of the ring. This allows significantly less thanthe entire outer circumference of the hanging rings to be in contactwith the inner surface of the drill collar. Thus, the rings will moreefficiently attenuate the vibrational force of the acoustic pulsescoming in contact with the hanging ring. The plurality of heavy hangingrings are spaced and sized for the maximum attenuation of acousticpulses in a predetermined range, preferably in the range of 10 khz to 20khz. The system may include steel rings as the plurality of heavyhanging rings. In an alternate embodiment, the plurality of heavyhanging rings may be a heavier, more dense material such as tungsten.The plurality may have as many as ten rings or as few as six, with eightbeing another possibility. The spacing of the rings may vary betweentwelve and fourteen centimeters, depending on the material used. In astill further embodiment, a pipe may be placed within the innercircumference of the rings to isolate the attenuation rings from theflow of drilling mud. The isolation pipe may be of any material,however, a material that is non-rigid that is less likely to conductvibrational forces is preferred. In another embodiment of the invention,the mass discontinuities are attached to the drill collar over asubstantial portion of their individual axial lengths. Such anarrangement acts as a low pass filter. When this mechanical arrangementis used with an electrical bandpass filter in the tool, high frequenciesare efficiently attenuated. In yet another embodiment of the invention,the attenuator section comprises a cylindrical body with sections ofdifferent inside and/or outside diameters to produce a ringed pipe: thesections of different diameter each have a characteristic passband and areject band for attenuation of signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a drill system having a measuring whiledrilling device mounted in the drilling apparatus.

FIG. 2 illustrates raypaths of acoustic signals between the transmitterand the receiver.

FIG. 3 is an illustration of an attenuation system for use on a welldrilling collar.

FIG. 4 is a graphical representation illustrating the effects of anincreased number of attenuation elements of a system as that illustratedin FIG. 1.

FIG. 5 is a graphical representation illustrating the effects ofincreasing the weight of attenuation elements of a system as thatillustrated in FIG. 1.

FIG. 6 is a graphical representation illustrating the attenuation effectof the system of FIG. 1.

FIGS. 7 a and 7 b show a comparison of the invention of FIG. 2 with onein which the mass discontinuities are attached to the drill collar overa substantial length.

FIGS. 8 a–8 c show alternate embodiments of the invention in whichattenuation is accomplished by means of recesses that produce massdiscontinuities in a body of the attenuator.

FIG. 9 shows a comparison of frequency spectra of attenuators havingdifferent types of recesses having a fixed length.

FIG. 10 shows alternate embodiments of the invention in which thediameter of the attenuation sections is varied.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a system and method for attenuatingacoustic waves in a down hole tool that is being used to obtaininformation about subsurface formations, some of which are believed tobe holding hydrocarbon deposits. FIG. 1 is a schematic illustration ofthe use of a Measurement-While-Drilling (MWD) apparatus while drilling awell. At the surface of the earth 5 a drilling rig 1 is used to drill aborehole 23 through subterranean formations 25 a, 25 b, 25 c etc. Thoseversed in the art would know that a drillship or a platform could beused to drill a borehole into subterranean formations covered by a bodyof water. A drilling tubular 13, that could be made of drill pipes orcoiled tubing is used to rotate a drillbit 17 at the bottom, therotating action of the drillbit and axial pressure carving out theborehole. When coiled tubing is used for the drilling tubular, adrilling motor (not shown) is used to impart the necessary rotary motionto the drillbit.

A variety of transducers are used downhole in a sensor assembly 11. Thissensor assembly makes measurements of properties of the formationsthrough which the borehole is being drilled. These could includeelectromagnetic, gamma ray, density, nuclear-magnetic resonance, andacoustic sensors. For illustrative purposes only, an acoustictransmitter array 31 and an acoustic receiver array 33 are indicated.Those versed in the art would recognize that other configurations of theacoustic transmitters and receivers could be used.

Turning now to FIG. 2, the transmitter 31 and the receiver 33 are showninside the borehole 23. The annulus between the drilling tubular 13 andthe borehole 23 is filled with a drilling fluid. The fluid is conveyeddown the borehole inside the drilling tubular to the drillbit andreturns up the hole via the annulus. Excitation of the transmitterproduces acoustic signals. A portion of the signal, denoted by theraypath 43, is referred to as the direct arrival and travels through thetool to the receiver. The transmitter also produces an acoustic signalin the borehole fluid that enters into the formation. One portion of it,illustrated by the raypath 41 travels as a body wave through theformation and carries information about the formation that it traverses.The receiver also detects other signals, such as tube waves that involvea coupled wave between the fluid and the formation, Stoneley waves thatare surface waves in the fluid, and signals reflected from acousticreflectors within the formation.

In an MWD tool, as in wireline tools, the body wave 41 through theformation usually arrives before the tube wave and the Stoneley wave.However, in an MWD tool, the direct arrival 43 through the tool commonlyarrives before the desired signal component 41 that carries informationabout the acoustic properties of the formation. In addition, thedrillbit 17 itself is continuously generating acoustic signals travelingthrough the drilling tubular 13. Consequently, it becomes very difficultto determine a travel time for the formation body wave 41.

In order to attenuate the direct arrival 43, the tool a pulse attenuator40 is located in tool 11 between transmitter 31 and an receiver 33. Onlyone transmitter and receiver are illustrated for demonstration. Inpractice, there may be several receivers and transmitters and thepresent invention operates with any arrangement, the only requirement isthat attenuator 40 be located between the transmitter and the receiver.

In one embodiment of the invention, the acoustic isolator is based uponan array of mass rings attached to the inner wall of the drillingcollar. Such an array presents an interference filter providing a stopband at a predetermined frequency for longitudinal sound wavespropagating along the walls of a collar. The device exhibits sufficientdamping within the predetermined frequency range as well as goodmechanical strength. The efficiency of an isolator of this typeincreases proportionally to the number of the rings N as well as to theratio M/μ, where M is the mass of a single ring, μ is a mass per unitlength of the collar. Hence, the efficiency of the isolator is verysensitive to even minor changes in outer dimensions of the pipe as wellas to the changes in demands to its wall thickness.

The attenuation provided by the isolator section is designed to be minusforty decibels within the frequency range of twelve through eighteenkilohertz. The isolator design satisfy the mechanical requirementsspecified concerning the limitations on inner diameter, outer diameter,minimal cross section area and others.

FIG. 3 is a partial illustration of an attenuation system 50 for a soundtool (not shown) in a drill collar 52 using an array of hanging massirregularities 54, 56, 58 . . . (may include up to ten elements) mountedon inner wall 60 of drill collar 52. Mass irregularities 54, 56, 58, . .. are secured to inner wall 60 by neck pieces 62 which extend out fromouter circumference 64, 66, 68, . . . of mass irregularities 54, 56, 58,. . . respectively. Neck pieces 62 are smaller both in depth and widththan outer circumferences 64, 66, 68, . . . of mass irregularities 54,56, 58, . . . so that mass irregularities 54, 56, 58, . . . are heldfirmly against inner wall 60, but not so firmly that acoustic pulsestraveling through drill collar 52 are transferred without attenuation.In this manner, mass irregularities 54, 56, 58, . . . are held firmlybut not tightly.

In an alternate embodiment, an inner pipe 64 may be provided to protectarray of mass irregularities 54, 56, 58, . . . from mud flow. Inner pipemay be of any material to isolate mass irregularities 54, 56, 58 . . .from the mud flow, however, a material that is non-rigid and has adegree of flexibility is preferred. A material that is less likely totransfer acoustic pulses toward the receivers is desired.

The operation of the attenuation filter may be understood by thefollowing discussion. The attenuator section has N mass irregularitiesor elements, each element having the shape of rings or donuts attachedto the inner surface of a pipe at the points x=x_(j), (where j=1, . . .n). The origin of coordinates coincides with the first irregularity,i.e. x₀=0. The mass of a ring j is m_(j). The distance between twoneighboring elements is:l _(j) =x _(j+l) −x _(j).

At x>x_(n), an incident longitudinal sound wave of a unit amplitudetraveling towards the origin of coordinates may be denoted bype ^(−i[k(x−x) ^(n) ^()−ωt])where

-   -   k=ω/c is a wavelength constant,    -   ω=2πf is an angular frequency,    -   c=the velocity of sound.

Due to the presence of an array there exists (at x>x_(n)) a reflectedwave p_(r)=V_(n)(ω)e^(ik(x−xn)−iωt), where V_(n)(ω) is a reflectioncoefficient for an array of n irregularities. In the present invention,the dimensions of irregularities are small as compared with the wavelength at a given frequency ω=2π/k. The density ρ as well as linear massof a pipe μ are also of great importance in the attenuation. In thepresent invention, the mass m_(j) is much greater than μh_(j), whereh_(j) is the length of attachment zone for the mass m_(j). Such an arraypresents an interference filter providing a stop band at a predeterminedfrequency range for longitudinal sound waves propagating in the walls ofa pipe.

In the solution of a wave equation, the length of a contact zone, Δl,between a ring and an inner wall of a pipe is small as compared to thewavelength of interest λ. Under these circumstances the propagation ofthe longitudinal wave can be described by the following differentialequation:

$\begin{matrix}{{{{YS}\frac{\partial^{2}u}{\partial x^{2}}} - {\mu\frac{\partial^{2}u}{\partial t^{2}}} - {M_{j}\frac{\partial u^{2}}{\partial t^{2}}{\delta\left( {x - x_{j}} \right)}}} = 0} & (1)\end{matrix}$Where:

-   -   Y is the Young's modulus of the pipe material,    -   S is the cross section area of the pipe wall,    -   u is the displacement,    -   μ is the linear mass of the pipe, and    -   x is the longitudinal coordinate.

When considering propagation of a sinusoidal wave, the displacement umay be represented by a function of the form u(x)exp(−iωt), where, ω isthe angular frequency, The differential wave equation then takes theform:

$\begin{matrix}{{{{YS}\frac{\partial^{2}u}{\partial x^{2}}} + {\mu\;\omega^{2}u} + {M_{j}\omega^{2}u\;{\delta\left( {x - x_{j}} \right)}}} = 0} & (2)\end{matrix}$For an array of N mass irregularities, the solution takes the form

$\begin{matrix}{{u(x)} = {{A\;{\mathbb{e}}^{{\mathbb{i}}\;{kx}}} - {\sum\limits_{j = 1}^{N}\;{b_{j}{G\left( {x - x_{j}} \right)}{u\left( x_{j} \right)}}}}} & (3)\end{matrix}$where,

-   -   A is an initial wave amplitude,    -   G(x−x_(j))=exp (ix/x−x_(j)/)/(2ysk) is Green function, and    -   b_(j)=M_(j)ω is the magnitude of an irregularity.        Hence the transmission coefficient at a position x that is        greater than x_(n) can be found as: T=u(x)/A, which may be        expressed in decibels using the usual conversion factor.

The transmission coefficient of the array may also be obtained by othermethods. One such method is an impedance approach, the relative inputimpedance is given by the formula:Z _(in)=(p/νρc)where:

-   -   p=pressure,    -   c=velocity of sound in the medium,    -   ν=vibrational velocity, and    -   ρ=density.

For an array of N elements, the impedance is calculated with the help ofthe following recurrence procedure:

${Z_{in}^{j + 1} = {\frac{Z_{in}^{j} - {i\;{\tan\left( {kl}_{j} \right)}}}{1 - {i\; Z_{ɛ}^{j}{\tan\left( {kl}_{j} \right)}}} - {i\frac{k\; m_{j}}{\mu}}}},{j = 1},2,\;{\ldots\mspace{14mu} N}$

FIGS. 4 and 5 illustrate plots of transmission vs. frequency. Theinfluence of the number of elements is illustrated in FIG. 4.Transmission curves are shown for six elements, eight elements and tenelements. The increase in the number of elements only slightly changesthe transmission curve at the borders of the predetermined frequencyband. However, the attenuation values of the transmission curves in themiddle of the frequency band are greatly affected. The period of anarray 1 is important to place the transmission curves at the properfrequency. In the preferred embodiment an optimal value for the spacingbetween elements is 5.12 inches or approximately thirteen centimetersfor the inner and outer diameter used. However, other spacings such asfourteen or twelve centimeters may also be used and provide acceptableresults. The influence of the mass of a single element is illustrated inFIG. 5.

FIG. 4 illustrates attenuation curves for arrays of ten elements. Eachcurve is for elements of different weights. A first curve is for tenelements, each weighing eight kilograms, the second for elementsweighing eleven kilograms and a third for elements weighing fourteenkilograms. An increase in the mass M results in changing the lowfrequency border. The high frequency border remains essentiallyunchanged. All the transmission curves show that transmission lossexceeds forty decibels within the predetermined frequency band betweentwelve and eighteen kilohertz.

The calculations were performed for an array of N identical equallyspaced irregularities. Transmission coefficient was calculated vs.frequency within the frequency range from five to twenty kilohertz.

FIG. 6 is a graphical representation of the attenuation of a preferredembodiment of the present invention. In the preferred embodiment, tenelements were used with a spacing of thirteen centimeters betweenelements. Rings of stainless steel were used as mass irregularities 54,56, 58 . . . . It can be seen that the arrangement of the preferredembodiment provides attenuation of waves in the range of eight toeighteen kilohertz. By using his system, interference of waves travelingthrough the collar of a drilling tool can be greatly reduced andacoustic logging is possible during a drilling operation.

FIGS. 7 a and 7 b show a comparison between the embodiment discussedabove with respect to FIG. 2 and an alternate embodiment of theinvention using a different arrangement of attaching the massdiscontinuities to the drill collar. Shown in the upper portion of FIG.7 a is a drill collar 152 a to which a mass 154 a is attached by meansof a neck 158 a. This corresponds to the arrangement discussed abovewith reference to FIG. 2. Shown in the upper portion of FIG. 7 b is analternate arrangement in which a mass 154 b is attached to the drillcollar 152 b over substantially the full length of the mass. Shown inthe lower portion of FIG. 7 a is a schematic representation of theeffective mass discontinuity 170 a as seen by a propagating wave:typically, such a mass discontinuity provides approximately 6 to 8 dB ofattenuation of the wave. The lower portion of FIG. 7 b shows theeffective mass discontinuity 170 b as seen be a propagating wave:effectively, an attenuation of 2–3 dB of attenuation is provided at eachboundary. By an analysis such as discussed above with respect toequations 1–4, the arrangement of FIG. 7 b is shown to act as a low passfilter. By suitable choice of the spacing and size of the weights, theeffective cutoff frequency can be made to be around 10 kHz. When used incombination with an electrical bandpass filter (not shown) on the tool,body waves through the drill collar may be effectively attenuated.

FIGS. 8 a–8 c show alternate embodiments of the invention in which theisolator comprises a machined cylindrical member. In FIG. 8 a, thecylindrical member has an outer diameter of OD and an inner diameter ofID. The inner diameter allows passage of drilling mud. The inside wallif the cylindrical member has recess of length L therein. A body waveencounters regions of different cross sectional areas and massdensities, similar to the embodiments discussed above, resulting inattenuation of body waves.

FIG. 8 b shows an arrangement in which the recess are on the outside ofthe isolator whole FIG. 8 c shows an arrangement in which there arerecess on both the outside and the inside of the isolator.

FIG. 9 shows the results of a finite element (“FE”) simulation of thevarious embodiments shown in FIGS. 8 a–8 c. The abscissa is thefrequency and the ordinate is the normalized amplitude of waves passedby the attenuator. Note that the amplitude scale is linear, rather thanbeing in decibels. The curve 301 shows the spectrum for a cylindricalpipe. The curve 303 shows the spectrum for cuts on the inside of thepipe, 305 is for recesses on the inside and outside of the pipe while307 is for recesses on the outside of the pipe. Similar FE simulationshave been carried out for various lengths L of the recesses. Based uponthese simulations, for an OD of 7.09″, in a preferred embodiment of theinvention, a value of L of 3.15″ (8.5 cm) with recesses on both theinside and the outside of the isolator is used.

The results in FIG. 9 are for a plurality of equally spaced recesseshaving the same length and the same depth of the recesses. Otherembodiments of the invention use a combinations of sections havingdifferent lengths and different depths of inner and outer recesses.Examples are shown in FIG. 10. Each section 400 may be considered to bea waveguide with an associated pass-band and a reject band determined bythe inner diameter 403 and the outer diameter 401. As may be seen inFIG. 10, each section has an axis parallel to the longitudinal axis 405of the body of the attenuator. By using such a combination of differentinner and outer diameters, a broad range of frequencies may beattenuated. This attenuation is in addition to the attenuation producedby reflections between adjacent sections 400. In the presence ofborehole fluid on the inside and outside of the sections, the waveguidesare “leaky” waveguides that allow energy to propagate into the fluid. Ina preferred embodiment of the invention, the inner diameters range from2″ to 6″ and the outer diameter ranges from 4″ to 10″.

While there has been illustrated and described a particular embodimentof the present invention, it will be appreciated that numerous changesand modifications will occur to those skilled in the art, and it isintended in the appended claims to cover all those changes andmodifications which fall within the true spirit and scope of the presentinvention.

1. A system for attenuation of acoustic waves traveling through alongitudinal member capable of transmitting said acoustic wavestherethrough comprising: a plurality of spaced-apart masses firmlyattached to an adjacent inner wall of said longitudinal member, eachsaid plurality of masses having a predetermined spacing and apredetermined maggnitude for attenuation of acoustic pulses in apredetermined frequency range.
 2. The system for attenuation of acousticwaves according to claim 1 wherein said plurality of masses comprises amaterial selected from (i) steel rings, and, (ii) tungsten rings.
 3. Thesystem for attenuation of acoustic waves according to claim 2 whereinsaid plurality of masses is between six and ten.
 4. The system accordingto claim 1 further comprising a center pipe fitting closely against aninner periphery of said masses for preventing contact between a boreholefluid and said plurality of masses.
 5. The system according to claim 4wherein said center pipe is constructed of rubber.
 6. The systemaccording to claim 1 wherein said spacing of the masses is within therange of twelve to fourteen centimeters.
 7. The system according toclaim 1 wherein the masses comprise metal rings attached to the innerwall of the longitudinal member by neck pieces extending outward from anouter circumference of the rings.
 8. The system according to claim 1wherein each of said plurality masses is attached to the longitudinalmember by at least one neck piece.